1. Field of the Invention
The present invention relates to information recording medium of recording and reproducing information by irradiation of a beam such as laser light, and a technology of recording information in the recording medium.
2. Description of the Related Art
There are known optical information recording media serving as memories of large-capacity and high-density. As one of such recording media, there is known a medium provided with a film made of a material capable of changing the phase between amorphous state and crystalline state. The film is attached to a substrate, as a recording layer. Information is recordable in the recording medium by thermal energy due to irradiation of laser light. There are known two types of the media: one is a write-once-read-many type (hereinafter, simply called as “WORM type”) in which information is recordable only once, and the other is an erasable type in which information is rewritable.
As the phase-changeable material used in formation of the recording layer of the WORM type medium, there is known an alloy film containing a Te oxide or a like compound as a main component, e.g., TeOPd-alloy. In the WORM type recording medium, information is recorded by formation of recording marks which are obtained by partially transforming the recording layer to a crystalline state. The crystallization is conducted by heating the recording layer to a crystallization temperature or higher.
As a phase-changeable material used in formation of the recording layer of the erasable type medium, there is known an alloy film containing Ge, Sb, Te, In, etc., as main components, e.g., GeSbTe-alloy. In the erasable type recording medium, information is recorded by formation of recording marks which are obtained by partially transforming the recording layer to an amorphous state, and the recorded information is erased by transforming the recording marks to a crystalline state. The recording layer is transformed into the amorphous state by heating the recording layer to the melting point thereof or higher, followed by rapid cooling. On the other hand, the recording layer is transformed into the crystalline state by heating the recording layer to such a temperature range between the crystallizing temperature and the melting point of the recording layer.
There is known a mark-length recording method, as a method of recording information in a medium. In the mark-length recording, marks of different lengths are formed between spaces of different lengths, so that each mark length and each space length (more specifically, the positions of the frontal edge and the tail edge of each mark) carry information.
In the mark-length recording, if a laser pulse of a strong intensity is irradiated in an attempt to form a long mark, temperature rise in a rear part of the mark is promoted due to heat generated around a frontal part of the mark, and as a result, a deformed mark having a small width at the frontal part and a large width at the rear part is formed, thereby degrading the signal quality of the mark. In view of this, it is advantageous to employ a method, as shown in FIG. 17. Specifically, in FIG. 17, modulated laser light is irradiated in a waveform of a pulse train comprising a multi-pulse section 90, and an off pulse 94 following the multi-pulse section 90 to form a mark 21, wherein the multi-pulse section 90 consists of a first pulse 91 corresponding to a frontal end 23 of the mark 21 which is formed along an information track 20, intermediate pulses 92 corresponding to an intermediate portion of the mark 21, and a final pulse 93 corresponding to a tail end 24 of the mark 21, and the off pulse 94 following the final pulse 93 has a power lower than that of the multi-pulse section 90. In FIG. 17, α, β respectively represent timings of generating the first pulse 91 and the final pulse 93, t1, t2, t3, and t4 respectively represent time lengths of the first pulse 91, the intermediate pulse 92, the final pulse 93, and the off pulse 94. The symbols “a”, “b”, and “d” respectively represent an intensity of laser light on the high-power side of the multi-pulse section 90, an intensity of laser light on the low-power side of the multi-pulse section 90 and the off pulse 94, and an intensity of laser light in a space 22.
According to the conventional recording method applied to the erasable type optical information recording medium, it is preferable to set the pulse widths t1, t2, t3, and t4 to possible lowest values depending on the linear velocity in recording within respective ranges of 0.5Tw≦t1≦2Tw, 0.4Tw≦t2=t3≦0.6Tw, 0.5Tw≦t4≦1Tw where Tw represents a reference clock cycle (also called as “window width”) of a signal to be recorded, and to select the pulse intensities “a”, “b”, and “d” in such a manner that the amplitude of the reproduced signal is not lower than a predetermined value (see Japanese Patent No. 3124720, called as “D1”).
There is known a drawback that as high-density recording is progressed, a small gap between adjoining marks 21 may adversely affect formation of the adjoining mark(s) 21 due to heat generated during formation of the target mark 21, and the edge positions of the marks 21 may be displaced to thereby degrade the signal quality. The displacement of the edge position due to heat interference between the adjoining marks 21 differs depending on the length of the mark 21 to be recorded, and the lengths of the spaces preceding and succeeding to the target mark 21. Therefore, in order to solve this problem, there is proposed a signal pattern adaptive recording compensation method of recording marks by flexibly changing the generation timing a of the first pulse 91 and the generation timing β of the final pulse 93 depending on the length of the mark 21 to be recorded, and the lengths of the spaces 22 preceding and succeeding to the target mark 21 (e.g., see Japanese Patent No. 2679596, called as “D2”).
Erasable type recording media have been developed because of convenience that the media are usable a number of times in light of the property that information is rewritable. However, a demand for development of WORM type recording media capable of recording information only once is also great in light of the property that information falsification is impossible.
In the WORM type information recording media, information is recorded by formation of recording marks which are obtained by partially transforming the recording layer to a crystalline state. The crystallization is carried out by heating the recording layer to or higher than the crystallization temperature, which is much lower than the melting point thereof. Accordingly, the information recording media of WORM type are likely to be susceptible to heat transfer from the preceding mark 21 which has been formed immediately before the mark 21 being formed, as compared with the recording media of erasable type in which the mark 21 is formed by rapidly cooling the recording layer after heating the same to or higher than the melting point thereof.
In view of the above aspect, there rises a drawback that jitter of reproduction signals is great in high-density recording conditions, if the conventional recording method as described above referring to FIG. 17 is applied to the recording media of WORM type using the recording layer made of e.g., TeOPD alloy. Thereby, the signal quality may be degraded, and precise information recording and reproduction may no be accomplished.